


Trust

by japanfan101



Category: Undertale (Video Game)
Genre: Established Relationship, Fluff, I know Gaster's supposed to be stuck in the void but in this series he got out, M/M, Post pacifist, Smut, Soul Sex, Undertail, but parts aren't mentioned so that's not the most important part of this story, that'll be explained in a later story, transguy oc
Language: English
Status: Completed
Published: 2016-01-23
Updated: 2016-01-27
Packaged: 2018-05-15 19:29:45
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 2
Words: 3,784
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5797000
Author URL: https://archiveofourown.org/users/japanfan101/pseuds/japanfan101
Summary: <blockquote class="userstuff">
              <p>In which Gaster and a human have some fun, monster style. ;)</p>
            </blockquote>





	1. Chapter 1

**Author's Note:**

> Hello! :D This is my debut story on this site. Not my first ever story, but… only my second soul sex story, and the first one I’m posting. (The whole soul sex thing is my favorite thing right now tbh) My OC, Kieran, is a transguy and in this oneshot he’s introduced to the way monsters get it on. There’s a bigger story about him and Gaster, but I want to figure out how exactly to write both their personalities and stuff. So this is sexy practice. Enjoy ;) Or don’t, that’s fine too. :p

The deep purple hue seeping into the room makes it seem as if we’d been teleported to some fairy tale world. I sit on the bed in awe, staring at the source of the light; Gaster’s soul. It’s a dark shade of purple mixed with some deep blue, and wisps of white were spread throughout the heart shape in my hands. There’s a vastness in his soul that draws me in, I feel like the whole universe is resting in my palm. _He’s… beautiful._ Sparks of light purple magic shoot out occasionally, startling me and causing the colors on the walls to change shade with every spark. My mouth is slightly open, ready to say something, and yet I can’t seem to form any words.

“Kieran? Are you still okay with this?” Concern laces Gaster’s voice, his face covered by a lavender blush. I nod, snapping out of my trance.

“Yeah, yeah, sorry… I just… Damn you’re incredible. Your… Your soul I mean. You’re incredible too though, I didn’t mean… Um…” I look away, embarrassed. “Sorry… I can’t seem to be able to English right now.” With a smirk I glance up at him, attempting to erase any awkwardness. His face is covered in purple by this point, and the violet flames in his eyes flicker.

With a deep breath he smiles softly at me, “I was becoming concerned… You were just staring, and I thought…” After a short pause, he finishes, his voice quieter than before, “I thought maybe you didn’t want to do this anymore.” His eyes won’t meet mine, and I can tell he’s worried.

I chuckle quietly and my smirk widens, “I was staring because this is new to me, and because… I’m in awe at how gorgeous your soul is, I never imagined anything like this before,” I lean down and press my lips against the side of his mouth, “but I love it. I love you.”

The blush on his face flares up and his smile grows. _Perhaps that’s a bit much, but Gaster looks happier and more at peace so I guess that was fine._ “So I’ll start… Uh…” I sit up and adjust the both of us, so that I’m sitting on his lap and facing him; his back is against the wall and his soul in my hands between us, “can I just…” I frown slightly, and blush creeps onto my face, “I’m just gonna go for it, then. Let me know if I fuck up, okay? Stop me if anything I do is… bad. Or something. The last thing I want is to hurt you, so…” I swallow hard and decide to stop stalling.

Gaster, seemingly unable to say anything, just nods in response. Shakily, my right hand moves closer and I barely touch a finger to the surface of his soul, eliciting a small gasp from him. The flames in his eye sockets flicker and glow brighter than before, and a burst of magic flashes out from his soul, enveloping my hand for a second before fading. It feels like static electricity, and the feeling lingers. The surface of his soul is almost like touching water, as if I press hard enough my finger would go right through it to the other side.

My finger slowly and softly traces the edge of it and Gaster shudders beneath me, looking away as his face flushes darker and he presses a hand over his mouth, trying to muffle a moan. I raise an eyebrow and grin, “Oh no no no, doc. You don’t get to look away. I want to see your sexy face, okay? Keep your eyes on mine, or else I’ll stop. How’s that sound?” I poke his soul with my finger lightly, enjoying the way he jolts in response. His flickering eyes slowly meet mine, looking embarrassed.

Gaster moves his hand away from his face and, after a little bit of thought rests it on my back, pulling me closer to him in the process, “If that’s what you want…” I brush my eyes over his face, smirking as I reward him by pressing a second finger against his soul and running my fingers back up to the top, more insistent this time but still carefully. His back arches and he attempts (poorly) to stifle a groan. Magic shoots out of his soul and leaves my fingers tingling; his hand on my back tightens and grips at my shirt.

I slow my fingers and lighten my touch, but don’t let up entirely. My fingers continue running along the side of his soul, up and down, tantalizingly slowly. Gaster takes in deep, gasping breaths and is shivering. Some sort of lilac colored liquid starts to drip out of his mouth. Without thinking, I stop moving my fingers as a playful grin makes its way onto my face. I lean in and kiss him, sticking my tongue out to lick up the strange liquid. He squirms beneath me, eyes glowing brightly and widening in surprise. The delicious taste of black licorice fills my mouth and I hum contentedly.

“Kieran… We… we don’t know if that’s safe for-” A loud moan pours out of his mouth, and I see his eyes seem to turn a slight shade darker as I wrap my hand around the heart in front of me gingerly to quiet his protest.

“I couldn’t help myself, you look delicious~ Plus, as a scientist you should understand the importance of _experimentation_ ” I rub my fingers over his soul, pressing into it harder than before for emphasis. Desperately trying to keep himself quiet, I hear just a small whine escape his mouth. “Hey now, don’t hold back” With a wink and a seductive smirk, I run my fingers up along the curve of his soul. My left hand begins tracing small patterns on the other side of his soul and that earns me a loud, surprised cry as his grip on me tightens.

“Haah… Kieran, that feels so good...” Gaster whimpers. I happily continue, wrapping my fingers around his soul and brushing them over every inch. I want to feel all of him. He quivers and lets out short, loud moans with every touch.

My movements stop for a moment and I ponder something. Deciding to go through with it, perhaps against my better judgement, I say, “I’m going to try… something. Okay? If it hurts… tell me. But if I’m right, I think it’ll feel amazing.” He gulps and nods, face flushed and eyes flickering rapidly in anticipation. My finger hovers over his soul and I push it forward, but instead of stopping at the surface of his soul I press my finger into the center of it, ignoring the slight resistance, until the first joint of my finger is resting inside him, and hold it there, gauging his reaction.

Gaster immediately jumps beneath me and makes a strangled noise, his soul shooting out a large burst of magic. His eyes dim to almost black and then blink back to life, glowing brightly. I worry that maybe I went too far and hesitantly begin to pull my finger out, but sensing my hesitation he grabs my wrist and holds it there, looking at me with longing in his eyes. His voice is deeper than usual, husky, “Don’t… don’t you dare. That’s… Ooooh god it feels…” His head falls back against the wall and he groans softly, forgetting to finish his sentence.

Filled with confidence and determination, I smirk and experimentally rub my finger around the inside. Gaster practically melts beneath me and lets out soft, shaky breaths in between moans. It feels like my finger is submerged in a liquid, smooth, warm, and slightly sticky. Gaster’s mouth falls open and more lavender liquid drips from his mouth to his chin, his eyes unfocused. I chuckle at the sight.

_Someone as refined as him, falling to bits under my touch… heh. I could get used to this._

Gaster’s face is now completely purple, flushed, hands gripping the bedsheets. Soft mewls escape his mouth as I slowly caress the inside of his soul. I push a second finger in next to the first and am rewarded with a loud gasp and his back arching, with his eyes growing brighter.

My fingers continue to explore him slowly, but the way he’s writhing and calling out my name makes it hard to focus. I curl my fingers up and Gaster tenses up for a second, eyes flashing brightly, rapidly, “K-Kieran, please… please, please more…” He lets out a gasping cry as I alter the movements, hitting different spots in his soul. I continue, getting faster with each and every sound that pours from Gaster’s mouth.

After just a few moments of this, he goes completely rigid and I feel the burst of magic before I see it. The room explodes in a dark violet color and Gaster cries out beneath me one last time as he cums. At least… I think that’s what happens. I slow my fingers, but keep them moving oh so gently until the light fades.

As he goes limp, I pull my fingers out. I note, surprised, that he’s gasping for breath. _Odd… He doesn’t need to breathe, does he?_ I gently push his soul to his chest, putting it back in place. Gaster’s breath hitches in response to the slight touch. He pulls me right up against him afterwards, holding me close. I notice that my fingers aren’t wet at all. _Wonder what a soul is actually made of…_

With a smile, I nuzzle my head against his chest and wrap my arms around him as well. We don’t say anything for a while. His breathing slows, and eventually stops altogether.

My eyes have closed and I’m about to fall asleep when I hear him whisper something, “I love you.” His head rests atop mine and I smile.

I press myself tighter against him, “I… I love you too. Thanks for letting me do that. Thanks for trusting me with your soul.” _Considering everything, I’m surprised he’d trust any human to do that honestly._ Feeling him shift slightly, I readjust myself as well and get more comfortable.

“Of course I trust you. I have no reason not to” He sighs, contentedly. With an embarrassed laugh, he adds, “I’d like a bit more warning before you… uh…” He turns his gaze away, "Before you shove your finger into my soul. It wasn’t bad, but it did take me by surprise.”

A smirk dances on my face, “Aw, but you loved it! Your reactions were so damn cute. But yeah I’ll be more careful… Sorry. Curiosity got the better of me, I didn’t mean to make you uncomfortable. And honestly I was kinda at a loss, y’know? I mean I’m not that creative and I wanted you to enjoy it. I hadn’t seen a soul before, let alone touched one, so it’s… new to me. If you don’t want stuff like that to happen, you’ll have to guide me, okay?”

“I understand, but how about I show you instead of telling you?” There’s a hint of mischief in his voice, “aren’t you curious about how it feels, love?” His hand moves to rest against the center of my chest.

My face instantly heats up. “Uh… Um… H-how does another time sound? I don’t… I’m curious, yes, but I’m not going to expect that or ask that, plus I’m… I’m weird and I don’t know if I want to be so vulnerable…” I shift in his grasp, pressing my face against his neck. I’m not opposed to his offer at all, on the contrary my body heats up at the suggestion. But I figure he needs time to rest, plus I’m not ready to be that vulnerable around him... yet.

He hums softly into my neck, “it’s perfectly okay if you don’t want me to, but I’m happy to show you anytime. I would never hurt you.” His teeth press lightly against my neck in a mock kiss.

We adjust our position, and before I know it we’re laying down on the bed, holding each other. After just a few moments, we’re both asleep, wrapped up in each others' arms, smiles adorning our faces.


	2. Role Reversal

**Notes for the Chapter:**

> So after some thinking, I wanted to write more soul sex stuff! I figured it’d go well as a second chapter here, so that’s where I’m putting it. This time, Kieran’s receiving. ;) 
> 
> Honestly I'm happier with chapter 1 than with this one, but it's not bad I just don't like it quite as much. If anyone has suggestions for improvements, I'd love to hear them! ^^

I swallow hard, nervously looking away. Gaster gently rests his palm against my cheek and tilts my head up, forcing me to look him in the eyes. His eyes are focused on mine, a soft, kind expression on his face. “Kieran, you don’t have to be nervous. I’m not going to hurt you.” I feel my body relax at the sound of his soothing voice, and nod.

“I’m not nervous… I’m fine… I… Just don’t know what to expect, and that scares me.” A small shiver runs down my spine. “Well go ahead then, do the thing.”

With an amused look on his face, he chuckles, “Your wish is my command.” Gaster’s hand goes to my chest and I tense up, unsure of what to expect. Slowly, so slowly, he pulls his hand forward. I feel like something is tugging out of my core and gasp at the unexpected sensation. Afterwards, I feel like I’m floating. It’s strange, and incredibly surreal. I look at his hands and my eyes widen. The soul resting in his hands has a black outline, but inside light blue and pale yellow dance together and swirl around one another. In some places they mix, to create a really pretty shade of green.

“Whoa… That’s… That can’t be mine. It’s too… pretty. Too smooth...” I can’t keep from staring, I know that it’s my soul (after all, I just watched Gaster pull it out of me), yet it feels so odd.

His laugh is soft, quiet. “Of course it’s yours, and it’s quite beautiful. The soul is the culmination of one’s being, it holds everything you are and everything you were. Each person’s soul is different in appearance, strength, and a variety of other things. A good indicator of a person’s character is actually the colors in their soul and how they join together and mix, each combination holds a certain significance and when a person’s soul holds a color, that color shows what values they hold dear.”

“That’s pretty cool, actually.” I grin, excited that I learned something new about souls. I’d been curious from the beginning, but uncertain of how to ask or if it was even okay to ask.

Gaster watches me carefully and smiles, brushing a hand along my cheek. “If you need me to stop for any reason, please speak up. This is… quite intense. I’m unsure of how it compares to what humans do, but it’s very much possible that you may need a break now and again to catch your breath.”

I nod, face flushing. “Uh… Alright. Well I guess… go for it then.”

When Gaster’s finger lightly comes into contact with the surface of my soul, I find myself already having trouble drawing breath. At his initial touch, an electrifying sensation flows through me, which eventually dulls to a heat that spreads through the entirety of my body. A whimper escapes my lips and I grasp at the sheets, already feeling like I can’t last much longer. “Haah… G-Gaster, I…” Another surge of electricity flows through me before I can finish my sentence, drawing a groan out of my throat. He had moved his finger only slightly, and a playful smirk adorns his face at my reaction. “W-wait, it’s… How… How is it supposed to feel?” My face flushes a bright red, “Because my mind’s already going blank…”

“That’s perfectly all right, Kieran. I told you it would be intense.” His expression softens, “I can stop if you’d like, but I was hoping to let you experience it all.”

With a quiet sigh, I shake my head. “It feels so good, I just don’t know how… how long I can take it. P-please, though, please continue.” I try hard to sound nonchalant about it, but the knowing grin on his face tells me that he knows I’d beg for more if I weren’t so embarrassed. His finger presses down harder than before, and he rubs little circles into the top of my soul. As the first wave hits me, I cry out loudly from both the surprise and the intense heat enveloping me. The waves of pleasure don’t stop, and with each one they grow a tiny bit more intense. The pleasure feels like it’s spreading thoughout my body, but it’s more than that. The movement of Gaster’s fingers against my soul shake me to my very core. This is like nothing I’ve ever felt before, but it’s so much better than I could have ever imagined. My mouth is hanging open and my forehead is already covered in sweat. “Ooooooh fuck…~”. Each time his finger retraces a circle, I shudder and gasp. Soon my eyes are fluttering shut and my breathing goes completely ragged.

Somewhere, distantly, I hear Gaster’s quiet laugh. “It’s only been a few minutes, and I’ve hardly gotten started. There’s so much more to show you.” His finger slows and eventually stops, leaving me a shivering mess hardly able to keep my own eyes open. There’s a trail of saliva dripping down to my chin, and I feel the soothing warmth from his light, unmoving touch spreading through me again. “Your soul is amazing, warm, and the way your energy swirls around with it looks and feels quite wonderful.”

“Gaster…” I fall into him, clutching at his sweater.

A hint of concern could be heard clearly in his voice, “do you want me to st-“

“No, no no, please don’t stop…!” I’m breathing heavily, trying desperately to keep air in my lungs. A look of surprise crosses his face, perhaps at being cut off, but he smiles and does as I ask.

It feels like I’m on fire when he starts moving again, but slower this time. I feel every movement, every light brush, as his fingers dance around the surface, alternating between feather-light touches and nearly pressing inside my soul. I’m completely filled with warmth, and there’s electricity flooding through my entire being. I bite down on my bottom lip to stifle my moans, but with each and every stroke of his finger my control abandons me more and more.

Gasping for breath, I practically melt into him, whimpers pouring out of my throat as I grip his sweater more tightly.

His touches slow and I whine, not wanting him to stop. I feel a hand hold onto my lower back, keeping me in place. Gaster’s voice is low, soft and gentle. “Dear, this is going to be far more intense from here on. If you want me to continue, I need you to remember to breath, okay? Can you make sure to do that for me?” I just nod frantically, nuzzling closer to him.

I open my mouth to say something, but instead a moan makes its way up my throat, muffled by his sweater. It’s hard to know exactly what he’s doing between the mind-numbing feelings and the novelty of the situation, but my head goes blank as flames course through my veins. Remembering what he said though, I take short, shaky breaths between muffled cries.

When I hear Gaster’s voice, he sounds miles away. Every now and again, though, he tells me how great my soul feels, or sometimes reminds me to relax and breath. The waves of pleasure slam into me relentlessly, electricity crackling over my skin and lighting my nerves on fire. Each shockwave sends me a bit closer to the edge. I’m shivering, quaking in his arms and I bite down lightly on one of his ribs to try and muffle my cries further, which draws a gasp out of him.

Without much warning, I tense up and feel like I’m about to explode. I’m seeing stars as I cry out Gaster’s name, clinging to him and shaking violently. The sensations slow and I whimper, every breath I try to take catches in my throat and my heart beats don’t feel like they’ll ever return to normal.

After letting me ride out the aftershocks, two hands rest on my back, and start rubbing softly. “Here, love, I’m going to move you to put your soul back in. Is that all right?” Finally after taking a deep breath in I manage to nod, and feel him lean the both of us forward until I’m laying on my back. With slow, soft movements, he returns my soul to its place in my chest. One last spark passes through me at the contact and I shakily sigh.

Gaster lays down next to me and runs his fingers idly through my hair, and if it weren't for my heavy breathing, there'd be perfect silence filling in the room. My heart slows down, back to normal, and I no longer hear the loud pounding in my ears.

My eyelids feel heavy, my body feels weak, the warmth and relaxation not doing anything to help keep me stay awake. “Mmm…” I wrap my arms and legs around Gaster, nuzzling my face against his cheek with a satisfied smile on my face.

He chuckles quietly, but doesn’t say anything; instead he holds me tightly, pressing his forehead against mine softly. My chest continues to rise and fall heavily, but after some time I’m breathing normally again. Neither of us have fallen asleep yet, but the room is warm and comfortable.

Face flushing in embarrassment, I break the silence. “Gaster… I… I’m honestly really embarrassed. Um… That just felt… amazing… I couldn’t help myself. I... I thought I'd last longer...” I bury my face in the crook of his neck. “Also, p-please don’t remember my reactions or anything, I just… I can’t believe that I reacted… that way…”

There’s a hint of mischief in his voice as he responds with a sultry whisper, “I will never forget that. You were fantastic, and your reactions were the best part.”

Whining, I grip him tighter. “Noooooo it was too embarrassing…”

A gentle laugh comes from his mouth, and his voice is softer than before. “Kieran there’s no reason to be embarrassed. I love everything about you, have I not made that clear?” I don’t respond, instead I pout and nestle myself closer to him. Gaster’s whispers calm me, though. All the muscles in my body relax, and with a quiet hum I close my eyes.

“I love you too… nerd.” Since I’m half asleep, my words are somewhat slurred. It doesn’t take long for me to pass out, loosening my grip on Gaster but not letting go entirely.

**Author's Note:**

> I didn't know exactly how to end it, so the ending's kinda weird but hopefully not bad. Please let me know how I can improve, I need the input! Also if you have Gaster headcanons, feel free to send 'em my way! I've got my own but I want to mess with new ways to write him, experimentation is key~


End file.
